1. Field of the Invention
The present invention relates to a wet-type image formation apparatus that forms a toner-fixed image using a liquid developer including a carrier liquid and toner particles.
2. Description of the Related Art
In an image formation apparatus employing an electrophotography method, an electrostatic latent image formed on a photoconductor using a developing device is developed by toner. A toner image, which is formed by the toner adhering to the electrostatic latent image, is transferred and fixed onto a recording medium. In this way, the toner-fixed image is formed. Conventionally and generally used in the image formation apparatus employing the electrophotography method are: a dry-type developing method employing powder toner; and a wet-type developing method employing a liquid developer.
Such an image formation apparatus of the electrophotography method is required to efficiently form a toner-fixed image, which is finally formed, with less distortion in the toner-fixed image. To attain this, conveying means for electrostatically adsorbing and conveying the recording medium is employed to prevent displacement of the toner-fixed image and achieve efficient image formation.
Examples of a document disclosing an image formation apparatus employing such conveying means include Japanese Laid-Open Patent Publication No. 11-265131, Japanese Laid-Open Patent Publication No. 2001-356613, and Japanese Laid-Open Patent Publication No. 2001-305868.
In a dry-type image formation apparatus employing the dry-type developing method as disclosed in Japanese Patent Laying-Open No. 11-265131, a toner image is transferred onto a recording medium with the recording medium being electrostatically adsorbed to the conveying means. In doing so, the conveying speed is changed depending on a type of toner image, and the transferred toner image is fixed onto the recording medium using only a pressurizing and heating mechanism.
In a wet-type image formation apparatus employing the wet-type developing method as disclosed in each of Japanese Laid-Open Patent Publication No. 2001-356613 and Japanese Laid-Open Patent Publication No. 2001-305868, a toner image is simultaneously transferred and fixed onto a recording medium with the recording medium being electrostatically adsorbed to the conveying means.
In recent years, an image formation apparatus used for an office printer, an on-demand printing apparatus, or the like for massive printing is required to attain higher efficiency, higher image quality, and higher resolution. To achieve this, a wet-type image formation apparatus has begun to be used which employs a liquid developer including toner particles having size smaller than that of toner particles of powder toner employed in a dry-type image formation apparatus and provides less distortion in the toner-fixed image.
The liquid developer used in the wet-type image formation apparatus includes toner particles and a carrier liquid. In the wet-type image formation apparatus, an electrostatic latent image is developed using this liquid developer to form a toner image, which is then transferred and heated to fix it onto a recording medium. In this way, a toner-fixed image is formed.
However, if an excess of carrier liquid is left in the toner image when heating and fixing the toner image after the transfer, the toner particles are less likely to be melted and combined with each other, with the result that the toner image is fixed on the recording medium at a low strength and the toner-fixed image is therefore likely to be detached. Moreover, the particles not sufficiently melted provide roughness in the image portion, thus resulting in a toner-fixed image having a low degree of transparency. Meanwhile, when heating and fixing under application of pressure, the following phenomenon takes place: toner particles not sufficiently melted and combined with each other adhere to a fixing member. In particular, this phenomenon is likely to take place when the fixing member is set at a low temperature and the carrier liquid is less likely to be volatilized. Hence, the phenomenon is called “low-temperature offset”.
On the other hand, even in the case where there is a small amount of carrier liquid when heating and fixing the toner image after the transfer, the carrier liquid remaining on the surfaces of the toner particles prevents the toner particles from being combined with each other. Accordingly, the toner image is fixed onto the recording medium at a low strength and the toner-fixed image is likely to be detached and scratched. Moreover, roughness is left in the image portion, thus resulting in a toner-fixed image having a low degree of transparency. When heating and fixing it under application of pressure, bonding strength between the toner particles is not sufficient, with the result that part of melted toner particles are divided by the fixing member at a boundary portion between the toner particles. This results in the following phenomenon: the divided toner particles adhere to the fixing member. In particular, this phenomenon is likely to take place when the fixing member is set at a high temperature and the carrier liquid is likely to be volatilized. Hence, the phenomenon is called “high-temperature offset”.
Here, none of the image formation apparatuses disclosed in Japanese Laid-Open Patent Publication No. 11-265131, Japanese Laid-Open Patent Publication No. 2001-356613, and Japanese Laid-Open Patent Publication No. 2001-305868 sufficiently considers suppression of the low fixing strength and the offset phenomena including the low-temperature offset and the high-temperature offset.